1. Field of the Invention
This invention relates to pre-engine converters. It more particularly refers to means for supplying the heat necessary to support the endothermic nature of their operation.
2. Description of the Prior Art
Pre-engine converters are known in the internal combustion engine art. Their purpose is to supply a high octane fuel to an internal combustion engine by converting a low octane number fuel in the fuel tank to light, normally gaseous hydrocarbons. Patents directed to this technology have been issued at least as early as 1940. Many different catalysts have been disclosed in the patent literature including various zeolites such as Y and ZSM-5. Pertinent patents include: U.S. Pat. Nos. 3,855,980; 3,635,200 and 2,201,965. There may be others but these three seem to represent the state of the art.
All of these references have in common the feeding of low octane fuel over a cracking catalyst to produce a product comprising a light, high octane gas. Explicit or implicit in the state of the art is the desire to exclude air (or other oxygen-containing fluids) from the conversion zone so as not to precombust fuel and therefore reduce the amount of fuel available for burning in the engine.
Most of the prior art recognize that two significant problems exist with pre-engine converters; cold start operation and catalyst regeneration. As to the first, hydrocarbon cracking is a substantially endothermic process requiring a continual source of heat to support catalytic cracking activity. In refineries, catalytic cracking is carried out at about 900.degree. F. The heat necessary to support cracking is often supplied by burning residual coke off catalyst. Thus in a fluid catalytic cracking process, hot, fresh or regenerated cracking catalyst is contacted with oil to be cracked. As the oil is cracked, it cools the catalyst and deactivates it by forming a layer of coke thereon. The cracked oil is separated from the coked catalyst which is then regenerated by burning off the coke which reheats the catalyst. These processes are continuous with very infrequent shut downs. Thus, there are few "cold starts." After a shut down, however, start up usually requires external heating of catalyst inventory at least to an extent necessary to initiate cracking so as to start the cycle. Thus, it is seen that in a refinery, supplying endothermic heat for the cracking reaction and regenerating the spent catalyst are complementary operations.
In a pre-engine converter, however, the cracking system employed uses a fixed catalyst bed rather than an FCC type system and is subjected to many, many cold starts, at least once a day on the average and in many cases substantially more often. Confronting these two problems within the constrains of operation set forth above and coupled with the fact that under cold start conditions, it is not even practical to attempt to burn off coke on catalyst to generate start up heat, the prior art has consistently suggested external heat sources. These include, obviously, electric heating with energy supplied by the car battery which may be quite suitable for cold starts and for running even after warm up. They have also included using the heat in exhaust gases (in the exhaust manifold for example), for running after the engine is warmed up. It has also been proposed to solve the catalyst regeneration problem by burning off the deposited coke with air. This usually took place in an operation separate from and alternating with converting. Thus, one suggestion was to use a swing reactor system with one catalyst bed converting and another regenerating, perhaps simultaneously. Another suggestion has been to operate one catalyst bed alternatively on conversion and regeneration cycles. Still, other suggestions have included using a pre-engine converter only during high load demand situations. Alternatively, proposals have been made to thermostatically control the operation of a pre-engine converter so that it only operated after the engine had warmed up to a sufficient extent.
While the generic concept of a pre-engine converter is on its face a desirable thing, in that it reduces the need for refining petroleum with its attendant yield losses and reduces or eliminates the need for octane appreciators, such as alkyl lead compounds, it has certain inherent deficiencies which have not yet been overcome. In particular, it is recognized by all that such a converter is a small in situ cracker which operates endothermically. Therefore, some means must be provided for operation under cold start conditions. The usual proposed solution to this problem is to heat the system electrically. Based on operating conditions of 900.degree. F and a space velocity of 20 LHSV, and assuming a warm up time of 1 minute, which is admittedly probably too long for practical application, the power input required would be 15 kw. This is too great a strain on the battery system of an ordinary passenger car as currently contemplated.
The alternative to this has been the proposal for a dual fuel system, a high octane one for start-up and a cruder fuel for steady state use. As a practical matter, this is an undesirable solution since it requires major modification of the fuel system existent in modern automobiles.
At noted above, catalysts for pre-engine conversions have been disclosed to be conventional cracking catalysts such as zeolite Y. Unconventional zeolite catalysts which have cracking activity, such as ZSM-5 zeolites, have been disclosed and claimed elsewhere for this use.